* Negotiators still have "lot of work" to reach accord
* Russia warns positions remain "far apart"
* Obama sees chance of deal, but has "no illusions"
* Iran, 6 powers aiming for final settlement by late July
* Uranium enrichment, Arak reactor at core of dispute
Positions between Iran and world powers diverge widely in some areas but Iranian negotiators seem "very committed" to reach an agreement on the country's disputed nuclear programme, a senior EU official said in an email seen by Reuters on Thursday.
Russia, one of the six major powers seeking to persuade Iran to scale back its contested atomic activities to deny it any nuclear bomb breakout capability, separately said the two sides were "far apart" on the issue of uranium enrichment.
The remarks underlined the uphill task confronting negotiators, who aim to hammer out a final settlement of the decade-old dispute over the nature and scope of Iran's nuclear programme in the next four months.
In a video message to mark the Iranian new year holiday Nowruz, U.S. President Barack Obama said there is a chance to reach a nuclear deal with Iran if it takes verifiable steps to assure the West the programme is for peaceful purposes only.
"I'm under no illusions. This will be difficult," Obama said. "But I'm committed to diplomacy because I believe there is the basis for a practical solution."
The brief email from European Union official Helga Schmid to senior officials of EU member states was written after a meeting between Iran and the United States, France, Germany, China, Russia and Britain in Vienna on Tuesday and Wednesday.
Schmid is the deputy of EU foreign policy chief Catherine Ashton, who is coordinating talks with Iran on behalf of the six nations. Tehran says its nuclear programme is peaceful but the West fears it may be aimed at developing the capability to make atomic bombs and wants it curtailed.
In this week's talks, Iran and the powers locked horns over the future of a planned Iranian nuclear reactor with the potential to produce plutonium for bombs, and the United States warned that "hard work" would be needed to overcome differences when the sides reconvene on April 7.
This line was echoed in Schmid's email.
"Since we are at an early stage of the final and comprehensive negotiations, we still have a lot of work ahead of us. On some areas, positions differ widely," it said.
"However, the impression is that the Iranian negotiators remain very committed to reach a comprehensive solution within the agreed 6-month period," Schmid added.
She was referring to a late July deadline for a long-term deal agreed in an interim accord struck in November.
The meeting in Vienna was the second in a series that the six nations hope will produce a verifiable settlement, ensuring Iran's nuclear programme is oriented to peaceful purposes only, and lay to rest the risk of a new Middle East war.
IRAN HAPPY ABOUT TALKS
The two sides sought to spell out their positions on two of the thorniest issues: the level of uranium enrichment conducted in Iran, and its Arak heavy-water reactor. Iran denies Western suspicions that it could be a source of plutonium.
Iranian Foreign Minister Mohammed Javad Zarif characterised the latest round of negotiations as "very successful" in terms of clarifying the issues involved, the Iranian official news agency IRNA reported.
"In terms of understanding and clarification, Vienna-2 was among our very successful round of talks ... extremely beneficial and constructive," it quoted Zarif as saying.
But Russia's chief negotiator, Deputy Foreign Minister Sergei Ryabkov, made clear that major hurdles lie ahead, in comments reported by Interfax news agency.
He said Iran and the powers agree that a solution should be based on November's preliminary agreement but that Iran had "very far-reaching demands" on enrichment, which can have both civilian and military purposes.
"The positions on this issue are far apart," Ryabkov said.
Under the interim accord, designed to buy time for talks on a long-term deal, Iran suspended higher-grade enrichment, a potential route to bomb-making, in exchange for some easing of sanctions that are battering its oil-dependent economy. But the powers want sharper cuts in Iran's overall enrichment capacity.
The U.N. nuclear agency, which has inspectors on the ground, was expected later on Thursday to issue a monthly update to member states showing Iran is living up to its part of the Nov. 24 agreement, diplomats in Vienna said.
Ryabkov said that great attention was paid to "issues of restrictions on Iran's enrichment activity and on Iran's prospects for enrichment activity in general", Interfax reported. "This is a very serious issue, which is very labour-intensive and causes many disputes," he said.
The next meeting of chief negotiators has been set for April 7-9, also in the Austrian capital. Expert-level talks will be held before then, officials say.
The over-arching goal is to transcend mutual mistrust and give the West confidence that Iran will not be able to produce atomic bombs while Tehran - in return - would win full relief from economic and financial sanctions.
Iran denies that its declared civilian atomic energy programme is a front for developing the means to make nuclear weapons. But its restrictions on U.N. inspections and Western intelligence about bomb-relevant research have raised concerns.